1. Field of the Invention
The object of this invention is an arrangement for controlling the output voltage pulses formed by a PWM frequency converter and more particularly the rising and falling edges of them.
2. Description of the Related Art
As is known in prior art a frequency converter forms output voltage of variable magnitude and variable frequency, which corresponds to the desired operating point of the motor connected to it, from a fixed-frequency supply voltage. The so-called PWM frequency converter has become established as the most general frequency converter type, in which the voltage of the supply network is initially rectified and filtered into the constant magnitude DC voltage of the intermediate circuit, from which the desired output voltage is then formed with fast power semiconductor switches (FIG. 1). The output voltage comprises pulses of the size of the constant DC voltage, the number and width of which are controlled so that the amplitude and frequency of the fundamental wave of the output voltage are those desired (PWM=Pulse Width Modulation).
In order to minimize the losses of the power semiconductor switches, they are generally controlled such that the on-controls and the off-controls occur as quickly as possible. In practice this means that the steepness of the rising edges and the falling edges of the output voltage pulses, i.e. the speed of change in the voltage dv/dt, is very large and dependent on the individually specific properties of the power semiconductor used.
As is known in prior art, a large speed of change in the voltage pulse has negative effects on the winding of the motor with both a short and a long motor cable:
1) The shorter the duration of the rising edge of voltage is, the higher is the proportion of the stress of the voltage step exerted on the first coil of the winding (see e.g. IEC's Technical Specification TS 60034-25, FIG. 12).
2) According to prior-art transmission line theory, a voltage pulse travels along a cable at a finite speed (approx. 50% of the speed of light), and a part of the pulses determined by the ratio of the wave impedances of the cable and of the motor are reflected back from the connection point. With a cable of the suitable length, owing to the reflection phenomenon the highest voltage pulse seen by the motor can be up to twice that of the voltage pulse sent by the frequency converter (see e.g. Transient Effects in Application of PWM Inverters to Induction Motors/Erik Persson/IEEE Transactions of Industry Applications, vol. 28 no 5, September/October 1992). The critical length of the cable, at the end of which the full-scale reflection occurs, depends on the time of duration of the rising edge of the voltage pulse; the faster the change is, the shorter the cable with which the full reflection occurs. For example the switching times of the IGBT transistors generally used as a power switch are of the order 0.1 μs, with which the critical cable length is approx. 30 m.
From the standpoint of the winding insulations of the motor, steep-edged and high voltage pulses are dangerous, as a result of which it is general to use filters implemented with passive components (inductance, capacitance, resistance) between the frequency converter and the motor, especially when supplying large voltages, in which the problem is at its worst. The general ones are e.g. dv/dt filters, with which the time of duration, and via that the critical cable length, is lengthened, and sine filters, with which the pulse-like voltage form is filtered to become almost sinusoidal for perfect elimination of the reflection problem. An example of a prior-art filter circuit is presented in FIG. 1 (without the damping resistors, which are used to prevent resonance oscillations of the filter). It is possible to influence with the magnitude of the inductance and capacitance values of the filter whether the filter operates as a dv/dt filter or as a sine filter.
A problem when using filters is their cost, size and weight. Especially sine filters are very large in size and expensive.